Default value for function argument, guaranteed to be unique - actionscript-3

Question

Just wondering, if i could provide default argument for function in ActionScript 3 which is never can be passed to the function by user. This is the case:
public function getAttr (obj:Object, key:String, def:* = DEFAULT_VALUE):* {
if (!obj.hasOwnProperty(key)) {
if (def === DEFAULT_VALUE) {
throw ReferenceError('Attribute not found: ' + key);
} else {
return def;
}
} else {
return obj[key];
}
}
I cannot use as DEFAULT_VALUE any of null, undefined, Number, Boolean or String here, cause, logically, user could use any of this values. I need something really unique here. In python, for example, i can do this:
_DEFAULT_VALUE = object()
def get_attr(obj, key, d=_DEFAULT_VALUE):
if not hasattr(obj, key):
if d is DEFAULT_VALUE:
raise KeyError('Attribute not found: {}'.format(key))
else:
return d
else:
return obj[d]
But in ActionScript 3 such approach produces an error:
Error code: 1047: Parameter initializer unknown or is not a compile-time constant.
Maybe some hack here?

Answer 1

Your problem here is that def value have to be a compile time constant so there is no way to store a default value into a var and pass it as a default value.
But what you really want here is to know if the user have passed an extra parameter into the def field of the function, so you can check the arguments array length and see if there is 2 or 3 parameters passed.
public function getAttr (obj:Object, key:String, def:* = null):* {
if (!obj.hasOwnProperty(key)) {
if (arguments.length==2) { // no default value passed to the function
throw new ReferenceError('Attribute not found: ' + key);
} else {
return def;
}
} else {
return obj[key];
}
}

Answer 2

Transfer null, and check for null. After all, null is a default value that's really nothing, if you require a parameter to be supplied and be valid, use null as default value so that if something will be supplied, it will not be null, thus valid.

Answer 3

Vesper's answer seems correct for your situation. But, I've used similar method for singletons though:
class Singleton
{
private var k_CONS_BLOCKER:Object = {};
public Singleton(cons_blocker:Object)
{
if(cons_blocker!=k_CONS_BLOCKER)
{
throw new Error("Use static getter to get the object");
}
}
public function getObject():Singleton
{
return obj;//you can of course create it now or do some other logic...
}
}
I know the answer isn't directly related to the question, but I think the idea solves the problem in general.
HTH.